Methods for producing an optical waveguide are roughly classified into thin film deposition methods and ion exchange methods.
The thin film deposition method is a method of depositing an optical waveguide layer having silica as a principal component on a substrate made of silicon, etc. Specifically, sputtering methods, CVD methods, flame deposition methods and the like are known. Disadvantageously, all these methods need high vacuum equipment for the production of waveguides and use complicated production processes, therefore resulting in increased costs. Also, in the CVD methods, flame deposition methods, etc., dangerous gases, such as SiH4, SiCl4 or the like, may be used, entailing high costs. Furthermore, the flame deposition method has disadvantages in that exposure to the high temperatures of about 1200° C. to about 1300° C. in the production process tends to degrade the substrate, and in addition causes internal stress in the substrate, increasing the polarization dependence of guided light, as well as other problems.
The ion exchange method is a method of using a multi-component glass which contains Na+ ions as a substrate and immersing the glass substrate in a molten salt containing K+ ions, Tl+ ions, Ag+ ions, etc. to exchange Na+ ions in the glass for K+ ions, Tl+ ions, Ag+ions or the like in the molten salt. Further, during the ion exchange, an electric field may be applied to increase the ion exchange rate, ion diffusion rate, etc. This method can raise the refractive index of the portion where ion exchange has been carried out, forming an optical waveguide layer. Unlike the thin film deposition method, the ion exchange method does not need a high vacuum, and the temperature of a molten salt is usually in the range of about 250° C. to about 400° C. Thus the production facilities are low-cost. However, it is necessary to strictly control the composition, temperature, etc. of the molten salt, which influence the rate of ion exchange, the rate of ion diffusion in the glass substrate, etc. Moreover, the temperature of ion exchange is influenced by the melting temperature of the molten salt. Therefore, when producing an optical waveguide having a desired refractive index profile by the ion exchange method using a molten salt, a high level of expertise is needed in the determination of ion exchange conditions such as the composition and temperature of the molten salt, processing time, etc.
Furthermore, since the ions introduced by the ion exchange method are monovalent cations, the ions used at present are limited to K+, Tl+, Ag+, etc., in almost all cases. However, these ions only serve to increase the refractive index of a waveguide portion. Therefore, an optical waveguide produced by introducing these ions only confines light in the waveguide layer and propagates the light from one side of the waveguide to the other. It is difficult to provide a waveguide with active optical functions, such as optical amplification, switching, etc.
The production of a waveguide having optically active functionality can be achieved by dispersing the ions, chemical species, etc. which can emit light with high efficiency or have the capability of changing the index of refraction with the intensity of light, that is, a high optical nonlinearity. Then, the emission of light enables optical amplification by induced radiation, and optical nonlinearity enables optical switching. One example of the ion with such characteristics is the copper(I) ion, and chemical species with such characteristics are copper(I) halide fine particles, copper(I) oxide fine particles, copper metal fine particles, etc. These are chemical species formed from monovalent copper ions or metallic copper produced by the reduction of copper ions; therefore, introducing monovalent copper ions into glass for the production of a waveguide makes it possible to provide the above-mentioned optically active waveguide.
In recent years, attempts have been made to produce an optical waveguide by introducing Cu+ ions (F. Gonella, F. Caccavale, L. D. Bogomolova, F. D'Acapito and A. Quaranta, J. Appl. Phys. 83, 1200 (1998); Jarmila Spirkova, Pavlina Nebolova, Ivan Jirka, Karel Mach, Vratislav Perina, Anna Mackova and Gabrila Kuncova, SPIE, Photonics West 2001, Conference 4277-47 (January 2001)). These references disclose that a molten salt of a copper(I) halide, a mixed CuSO4—Na2SO4 molten salt, etc. are used for introducing copper ions by ion exchange. However, when a copper(I) halide molten salt is used, ion exchange needs to be performed in a reducing atmosphere because Cu+ ions tend to be oxidized in air. When a mixed CuSO4—Na2SO4 molten salt is used, ions are exchanged after Cu2+ is reduced to Cu+, so that the rate of ion exchange is slow, and its control is difficult. Further, in a waveguide thus produced, monovalent copper ions and divalent copper ions are mixed, making it difficult to control the refractive index of the waveguide. Furthermore, the great difference in diffusion rate between the coexistent monovalent and divalent copper ions makes it extremely difficult to control the cross-sectional profile of the waveguide. Further problems also arise, including the difficulty of controlling the state of Cu+ present in the waveguide to obtain the desired optical characteristics.